1. Field of the Invention
The present invention relates to an apparatus having switchable servo gains and offsets for an optical disk device and the method thereof, and more particularly, to a servo control system of an optical disk device that makes adjustments in the gains and offsets according to the working status of the pick-up head.
2. Description of the Related Art
The more progressive information technology is, the more versatile the kind of data, storage is. In this regard, the optical disk drive has apparently become an important data storage tool available on the market. For an optical disk drive to operate, an electromechanical actuator drives a pick-up head so as to focus laser beams to be points on an optical disk. Then, any stored binary data is read and judged by the magnitude of the light rays received by a photo-detector; meanwhile, the received rays function as servo control signals for driving the pick-up head. In other words, the laser points are accurately focused on an adequate track, using such an optical signal to drive the pick-up head.
During the seeking process carried out by an optical disk drive, focus error (FE) signals is interfered with crosstalk that arises from the motion of the pick-up head along the radial direction of an optical disk. In other words, a carrier wave is induced on the FE signal from the interference of a tracking error (TE) signal. However, during the following or tracking process, the pick-up head moves from an inner portion to an outer portion along a spiral groove where data is recorded, thus the TE signal is almost inactive and the FE signal is free from the interference of the crosstalk effect on itself.
It is not feasible to adjust the gain of a conventional servo loop system for optical disk drives in the light of the working status of a pick-up head; thus, servo control is usually unstable, as it hardly converges. For instance, it is impossible to diminish the gain of the focusing loop system whenever seeking is underway; as a result, an inadequate response arises from that the interfering crosstalk signals are mistaken for normal FE signals. However, if the gain is set to a relatively small value, the response speed of the focusing loop control is compromised during the tracking process.
On the other hand, in regard to an ordinary optical disk, the reflectivity of a data area is different from that of a blank area. Hence, the photodetector of the pick-up head detects various reflective signals while it is passing the aforesaid areas. Data is saved in both the groove areas and the land areas of a DVD-RAM disk. Similarly, the reflective signals generated in the groove areas are different from those generated in the land areas where refractive indices are lower than the land areas' in terms of their characteristics. However, it is not also feasible to adjust the gain and the offset of the conventional servo loop system according to the position of the pick-up head where a data area or a blank area exists or where a groove area or a land area exists. By the same token, the servo control is unstable, as it hardly converges.